Voltage controlled oscillators are widely used in frequency reference applications such as the construction of phase locked loop devices and tuneable filters. For high quality applications where accurate frequency control is required, a piezo-electric, e.g. quartz crystal element is employed. Such crystals have very precise and stable frequencies. The disadvantage of such an arrangement is that of providing some means of frequency tuning. Conventional circuits that provide this function are somewhat complex and costly.
The object of the present invention is to minimise or to overcome this disadvantage.